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Olfaction

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The sense of smell is achieved through the activities of the olfactory system. It starts when an airborne odorant enters the nasal cavity and reaches olfactory epithelium (OE). The OE is protected by a thin layer of mucus, which also serves the purpose of dissolving more complex compounds into simpler chemical odorants. The size of the OE and the density of sensory neurons varies among species; in humans, the OE is only about 9-10 cm2.
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Humans detect odors with the help of specialized cells located in the upper part of the nasal cavity, called olfactory receptor neurons (ORNs). ORNs possess hair-like structures called cilia, which are receptive to sensations from the inhaled air. When an odorant molecule binds to a specific receptor on the cell of the cilia, it leads to a series of events that ultimately cause the ORN to send electrical signals to the olfactory bulb in the brain through the olfactory nerves.
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The process of olfaction, also known as the sense of smell, is a sophisticated chemical response system. The specialized sensory neurons that facilitate this process, known as olfactory receptor neurons, are situated in an upper segment of the nasal cavity, known as the olfactory epithelium. Olfactory sensory neurons are bipolar, with their dendrites extending from the epithelium's apex into the mucus that lines the nasal cavity. Airborne molecules, when inhaled, traverse the olfactory...

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Related Experiment Video

Updated: Jun 24, 2026

A Free-breathing fMRI Method to Study Human Olfactory Function

A Free-breathing fMRI Method to Study Human Olfactory Function

Published on: July 30, 2017

Trigeminal perception is necessary to localize odors.

A M Kleemann¹, J Albrecht, V Schöpf

¹Department of Neuroradiology, Ludwig-Maximilians-University of Munich, Germany. Anna_Maria.Kleemann@med.uni-muenchen.de

Physiology & Behavior

|March 24, 2009

Summary

Humans can detect but not localize odors that only stimulate the olfactory system, like hydrogen sulfide (H2S). However, they can localize odorants that also stimulate the trigeminal system, such as isoamyl acetate (IAA) and carbon dioxide (CO2).

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Published on: August 18, 2014

Area of Science:

Neuroscience
Sensory Perception
Olfaction

Background:

Human odor localization ability is inconsistently reported in scientific literature.
Distinguishing between olfactory and trigeminal nerve stimulation is crucial for understanding scent perception.

Purpose of the Study:

To investigate human sensitivity and localization abilities for olfactory-selective (H2S), olfactory-trigeminal (IAA), and trigeminal (CO2) stimuli.
To determine if odor localization depends on the type of sensory system stimulated.

Main Methods:

Utilized Signal Detection Theory to assess human sensitivity to H2S, IAA, and CO2.
Conducted localization experiments requiring conscious perception of stimuli.
Tested localization accuracy for varying concentrations of H2S and standard concentrations of IAA and CO2.

Main Results:

Humans demonstrated moderate sensitivity to low concentrations (2 ppm) and high sensitivity to higher concentrations (8 ppm) of H2S.
High sensitivity was also observed for IAA (17.5%) and CO2 (50% v/v).
Subjects could not localize H2S stimuli at any concentration, but successfully localized IAA and CO2.

Conclusions:

Conscious perception of a stimulus does not guarantee its localization.
Odorants solely activating the olfactory system are not localizable by humans, irrespective of concentration.
Odorants that co-activate the trigeminal system are localizable, indicating its role in spatial scent perception.